Geographic Methods in Geologic Investigation. 25 



The falls of such headwater sti'eams must persist until the 

 plateau is cut away, for the cap rocks over which the streams leap 

 being horizontal cannot be smoothed down till the whole plateau 

 is cut through. They are long-lived features. Moreover everj'- 

 one of the innumerable branch streams must on its way down 

 from the uplands fall over the outcropping edges of all the hard 

 beds. The falls will therefore be common as well as long-lived 

 features. Their frequent occurrence confirms the correctness of 

 this generalization. On the other hand, in regions of tilted rocks, 

 the hard beds are avoided by the streams, which select the softer 

 strata for their valleys. The hard beds soon stand up as ridges 

 or divides, across which only the large streams can maintain their 

 courses, and these are the very ones that soon cut down any fall 

 that may appear in their early stages. Falls on tilted rocks are 

 therefore rare not only because of their brief duration, but also 

 because tilted rocks are crossed by few streams, except the large 

 ones, which soon cut away their falls. 



The foregoing considerations show clearly enough that falls 

 like those of northeastern Pennsylvania are rare, and we have 

 now to consider why they should be prevalent in the region in 

 question. The Appalachians contain many water-gaps cut down 

 on tilted beds, every one of which may have been the site of a 

 fall for a relatively brief period of river immaturity, but this 

 brief period is now left far in the past. The streams show many 

 signs of maturity: then- slope is gentle and their valleys are wide 

 open from Alabama to Pennsylvania, but in the northeastern 

 corner of the latter State we find a group of streams that leap over 

 high benches into narrow gorges, and the benches ai"e held up by 

 tilted rocks. Manifestly the streams have in some way been 

 lately rejuvenated ; they have been, in part of their courses at 

 least, thrown back into a condition of immaturity, at a time not 

 long past, and, as has so well been shown by White, the cause of 

 this is the obstruction of their old channels by irregular deposits 

 of glacial drift. Here first in the whole length of the Allegheny 

 section of the Appalachians we find an exceptional condition of 

 stream life, and here also we come into a region lately glaciated, 

 where heaps of drift have thrown the streams out of their old 

 tracks. The explanation fits perfectly, and if it had not been 

 discovered by inductive observation in the field, the need of it 

 might have been demonstrated deductively. It is a case that has 

 given me much satisfaction from the promise that it holds out 



